In speech communication of a mobile communication system, communication using a monaural scheme (monaural communication) is a mainstream, such as communication using mobile telephones. However, if a transmission rate increases further as in the fourth-generation mobile communication system, it is possible to maintain an adequate bandwidth for transmitting a plurality of channels. It is therefore expected that communication using a stereo system (stereo communication) will be widely used in speech communication as well.
For example, considering the increasing number of users who enjoy stereo music by storing music in portable audio players that are equipped with a HDD (hard disk) and attaching stereo earphones, headphones, or the like to the player, it is anticipated that mobile telephones will be combined with music players in the future, and that a lifestyle of using stereo earphones, headphones, or other equipments and performing speech communication using a stereo system will become prevalent. In order to realize realistic conversation in the environment such as in currently popularized TV conference, it is anticipated that stereo communication is used.
Even when stereo communication becomes common, it is assumed that monaural communication will also be used. This is because monaural communication has a low bit rate, and a lower cost of communication can therefore be expected. Further, a mobile telephone which supports only monaural communication has a smaller circuit scale and is therefore inexpensive. Users who do not need high-quality speech communication will purchase mobile telephones which support only monaural communication. Accordingly, in a single communication system, mobile telephones which support stereo communication and mobile telephones which support monaural communication will coexist. Therefore, the communication system will have to support both stereo communication and monaural communication.
In the mobile communication system, communication data is exchanged using radio signals, a part of the communication data is sometimes lost according to the propagation path environment. Therefore, if the mobile telephone has a function of restoring the original communication data from the residual received data even in this case, it is extremely useful.
There is scalable encoding composed of a stereo signal and a monaural signal. This type of encoding can support both stereo communication and monaural communication and is capable of restoring the original communication data from residual received data even when a part of the communication data is lost. An example of a scalable encoding apparatus that has this function is disclosed in Non-patent Document 1, for example.    Non-patent Document 1: ISO/IEC 14496-3:1999 (B.14 Scalable AAC with core coder)